feat(studio): clearer store + channel switcher + approvals & welcome polish

- store: repair the filter dialog crash (a Spacer lived directly in
  AlertDialog.actions, which is an OverflowBar, not a Flex — it threw
  and rendered a broken dialog). Buttons now sit in a Row.
- store: promote the module-store manager from a bare icon to a
  labelled 'Add store' button, and fully localize the dialog (DE/EN).
- store: add a curated 'Suggested stores' shelf with one-click
  add/remove (first entry: Recl∆Im). Each suggestion is probed for
  reachability and shows 'not available yet' until its index is
  published, instead of failing only on click. Fail-open on network
  errors so a transient hiccup never hides a real store.
- sidebar: the channel pill is now a one-click channel switcher
  (menu with per-channel running state + active check; switching
  writes ~/.chain/current-channel, restarts the daemon, and Studio
  repoints to the new channel).
- approvals: lead the card with the human prompt ('what am I
  releasing?') and demote the flow/step id to a metadata line; clear
  fallback when the step left the prompt empty.
- welcome: tidy the docs grid into equal-height paired rows with a
  full-width trailing card for the odd one out.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/l10n/app_localizations.dart

@@ -545,7 +545,7 @@ abstract class AppLocalizations {
   /// No description provided for @sidebarChannelTooltip.
   ///
   /// In en, this message translates to:
-  /// **'Active channel — click to switch in Settings (⌘;).\nproduction = stable · beta = pre-release · dev = rolling · local = workspace'**
+  /// **'Active channel — click to switch.\nproduction = stable · beta = pre-release · dev = rolling · local = workspace'**
   String get sidebarChannelTooltip;
 
   /// No description provided for @buttonCancel.
@@ -1436,6 +1436,102 @@ abstract class AppLocalizations {
   /// **'Reload'**
   String get storeReloadTooltip;
 
+  /// No description provided for @storesManagerButton.
+  ///
+  /// In en, this message translates to:
+  /// **'Add store'**
+  String get storesManagerButton;
+
+  /// No description provided for @storesManagerTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'Module stores'**
+  String get storesManagerTitle;
+
+  /// No description provided for @storesManagerIntro.
+  ///
+  /// In en, this message translates to:
+  /// **'The hub merges the modules from every store below into one index. Add a store\'s index URL to make its modules installable here.'**
+  String get storesManagerIntro;
+
+  /// No description provided for @storesManagerAddSection.
+  ///
+  /// In en, this message translates to:
+  /// **'Add a store'**
+  String get storesManagerAddSection;
+
+  /// No description provided for @storesManagerNameLabel.
+  ///
+  /// In en, this message translates to:
+  /// **'Name'**
+  String get storesManagerNameLabel;
+
+  /// No description provided for @storesManagerUrlLabel.
+  ///
+  /// In en, this message translates to:
+  /// **'Index URL (https:// or file://)'**
+  String get storesManagerUrlLabel;
+
+  /// No description provided for @storesManagerAddAction.
+  ///
+  /// In en, this message translates to:
+  /// **'Add store'**
+  String get storesManagerAddAction;
+
+  /// No description provided for @storesManagerBundled.
+  ///
+  /// In en, this message translates to:
+  /// **'{count} modules · bundled'**
+  String storesManagerBundled(int count);
+
+  /// No description provided for @storesManagerModuleCount.
+  ///
+  /// In en, this message translates to:
+  /// **'{count} modules'**
+  String storesManagerModuleCount(int count);
+
+  /// No description provided for @storesManagerAddFailed.
+  ///
+  /// In en, this message translates to:
+  /// **'Could not add store'**
+  String get storesManagerAddFailed;
+
+  /// No description provided for @storesManagerRemoveFailed.
+  ///
+  /// In en, this message translates to:
+  /// **'Could not remove store'**
+  String get storesManagerRemoveFailed;
+
+  /// No description provided for @storesManagerSuggested.
+  ///
+  /// In en, this message translates to:
+  /// **'Suggested stores'**
+  String get storesManagerSuggested;
+
+  /// No description provided for @storesManagerAddShort.
+  ///
+  /// In en, this message translates to:
+  /// **'Add'**
+  String get storesManagerAddShort;
+
+  /// No description provided for @storesManagerNotAvailable.
+  ///
+  /// In en, this message translates to:
+  /// **'Not available yet'**
+  String get storesManagerNotAvailable;
+
+  /// No description provided for @storesManagerNotAvailableHint.
+  ///
+  /// In en, this message translates to:
+  /// **'The provider hasn\'t published this store index yet. It will become installable once they do.'**
+  String get storesManagerNotAvailableHint;
+
+  /// No description provided for @storesSuggestedReclaimDesc.
+  ///
+  /// In en, this message translates to:
+  /// **'Recl∆Im — modules for law & duty reform-mapping'**
+  String get storesSuggestedReclaimDesc;
+
   /// No description provided for @storeTodayBadge.
   ///
   /// In en, this message translates to:
@@ -2969,9 +3065,21 @@ abstract class AppLocalizations {
   /// No description provided for @approvalsPayloadPreview.
   ///
   /// In en, this message translates to:
-  /// **'PAYLOAD PREVIEW'**
+  /// **'DATA TO BE RELEASED'**
   String get approvalsPayloadPreview;
 
+  /// No description provided for @approvalsRequestFallback.
+  ///
+  /// In en, this message translates to:
+  /// **'Approval required for this step'**
+  String get approvalsRequestFallback;
+
+  /// No description provided for @approvalsFlowStepMeta.
+  ///
+  /// In en, this message translates to:
+  /// **'Flow: {flow} · Step: {step}'**
+  String approvalsFlowStepMeta(String flow, String step);
+
   /// No description provided for @approvalsApproveButton.
   ///
   /// In en, this message translates to: